Electrodes for electric discharge apparatus



Jan. 25, 1966 D. J. JONES ETAL 3,231,332

ELECTRODES FOR ELECTRIC DISCHARGE APPARATUS Filed July 31, 1962 l 44 8Fig.5 15 17 I 18 A 'N E I m v NTDKS :L1\\( '1 Hv D Joy/v x/ wFITTD'RNEYS United States Patent 3,231,332 ELECTRGDES FOR ELEQTRICDESCHARGE APPARATUS David John Jones, Harrow, Middlesex, David RichardWallis, Watford, and Wiliiam Roweil, Harefieid, Middlesex, England,assignors to The General Eiectric Company Limited, Victoria, London,England Filed July 31, 1962, Ser. No. 213,727 11 Claims. (Cl. 29182.2)

This invention relates to electrodes for use in electric dischargeapparatus, and is more particularly concerned with electrodes of thekind comprising an electron emissive part consisting mainly of tungsten,which, usually in the form of rods, find use, for example, in arcwelding and spark cutting apparatus and as cathodes in electricdischarge lamps. The invention also relates to electric dischargeapparatus in which one or more electrodes of the kind described is orare incorporated.

It has already been proposed to incorporate electron emissive materialsof relatively low work function, such as thoria and Zirconia, intungsten electrodes for use in electric discharge devices, since thepresence of the low work function material promotes rapid starting ofthe discharge at a relatively low temperature and low voltage. For someapplications, thoria is a particularly useful additive, on account ofits high electron emissivity, which enables an electrode incorporatingthoria to be operated over a wide range of values of electric current;moreover thoriated tungsten is less affected than pure tungsten by metalvapour pickup during operation of the electrode. The incorporation ofoxides such as thoria in tungsten, however, renders the materialdifficult to work, the workability decreasing and the brittleness of thematerial increasing with increasing oxide content. For some applicationsit is desirable to employ tungsten electrodes containing a highproportion of electron emissive material; for example for arc welding atlow pressures a thoria content of up to 15% may be required to giveadequate electron emission; tungsten containing such high proportions ofthoria is very brittle, so that the working of material of suchcomposition, to form electrodes of desired shapes, becomes extremelydifiicult.

It is an object of the present invention to provide an improved form ofelectrode for use in electric discharge devices, a particular objectbeing the provision of an electrode which is composed mainly of tungstentreated with electron emissive, low work function material, and whichcan be more easily formed by working the said treated tungsten than hashitherto been the case. Another particular object of the invention isthe provision of an improved form of electrode for use in arc weldingand spark cutting apparatus, especially for inert gas shielded tungstenarc welding.

According to the invention, in an electron emissive electrode for use inelectric discharge apparatus the electron emissive part consists of acore of tungsten containing a dispersion of electron emissive materialand, substantially surrounding the core and integral therewith, an outerlayer consisting essentially of tungsten and substantially free fromsuch electron emissive material, the material of the core having anappreciably lower work function than that of the material of the saidouter layer.

The outer layer may consist of pure tungsten, or may consist of tungstencontaining one or more additives of the kind which are not electronemissive but which influence the crystal structure of the tungsten,especially additives of the kind which promote the growth of largeelongated crystals, for example alumina and potassium silicate.

3,231,332 Patented Jan. 25, 1966 The outer layer of pure tungsten, or oftungsten con taining a large crystal growth-promoting additive, isrelatively ductile and hence, since it substantially surrounds the morebrittle core, renders the composite body more readily workable than thecore material alone would be. Thus the composite body may be worked, forexample by swaging and rolling, to a suitable shape and size of anelectrode for use in arc welding or cutting, or spark cutting apparatus,or of a cathode for an electric discharge lamp; usually the worked bodywill be in the form of a rod. If desired, the final shaping of theelectrode may be eifected by a machining operation, for examplegrinding.

It will be appreciated that for most applications, it will be necessaryfor a small part of the core to be exposed at the surface of theelectrode, to provide a surface capable of emitting electronsimmediately on the application of a voltage to the electrode, and it isto be understood that the phrase substantially surrounding the core,employed above in connection with the outer layer, includes the casewhere such small part of the core is exposed. For arc welding or sparkcutting applications, or for use as a cathode in an electric dischargelamp, the electrode is usually in the form of a rod consisting of a corecontaining the electron emissive material, surrounded longitudinally bysaid outer layer, the end surface of the core being exposed at least atone end of the rod. For these applications, in which the electricdischarge emanates from an end surface of the electrode, arising fromthe direct emission of electrons from the core, the thickness of theouter layer relative to the diameter of the core may be varied asdesired according to the relative requirements of current carryingcapacity, which is determined by the thickness of the outer layer, andrate of emission, which is determined by the exposed surface area of thecore: the ratio of the thickness of the outer layer to the diameter ofthe core is suitably in the range of 7:1 to 1:7. The electrode may be inthe form of a straight rod of constant diameter throughout its length,or if desired, especially in the cases of arc welding or spark cuttingelectrodes, the end at which the discharge is to be formed, that is tosay the end at which the core is exposed, may be tapered.

An electrode in accordance with the invention, when employed for any ofthe above-mentioned applications in which the discharge is produced atan exposed surface of the electrode, possesses the additional advantagethat control of the discharge is improved by the presence of thesurrounding outer, non-emissive layer. Thus the discharge area isconfined to the exposed area of the core, wandering of the dischargeaway from the exposed surface of the core being restricted by thepresence of the outer layer: this feature is of importance, for example,in arc welding electrodes and in electrodes for compact source dischargelamps.

If desired, for some applications, the electron emissive core of theelectrode may be wholly surrounded by the outer layer of tungsten, or oftungsten containing an additive as aforesaid, provided that at least apart of the said outer layer is sufliciently thin to permit theactivating atoms from the core to diffuse through it.

The electron emissive material employed in the core of an electrodeaccording to the invention may be any material known to be suitable forthis purpose, and capable of giving a core having an appreciably lowerwork function than that of the material employed for the outer layer,whether this is pure tungsten or tungsten contain-- ing one or moreadditives as aforesaid. Suitable elec-- tron emissive materials are, forexample, thoria, zirconia, and the oxides of yttrim, lanthanum and therare earth metals. As is well known, in the operation of an electrodecontaining one of these oxides, the oxide is first reduee'd to the metaland electrons are emitted from the metal atoms as they reach the surfaceof the electrode by diffusion, the steps of reduction, diffusion andemission taking place continuously during operation of the elec trode.Hence the proportion of emissive material incorporated in the core needonly be sufiicient to keep the surface from which emission is requiredto take place '(that is to say, the exposed area of the core or, if noneof the core is exposed, the whole of the emitting surface of thecathode) saturated with the electron-emitting atoms, for maintainingcontinuous emission at a suitable concentration. The amount of emissivematerial required 7 may vary according to operation conditions: forexample,

if thoria is employed as the emissive material, a thoria eontent in therange of 0.5 to by weight of the total weight of the core is usuallysufficient, but if the elect'rode is to be operated in an atmosphere atlow pressure, a considerably larger proportion of thoria, for example 10to may be required to maintain emission at the desired rate. I

In some cases where the outer layer contains additive material asaforesaid, it is not objectionable, and may even be desirable, toinclude in the core material, in addisome the electron emissivematerial, the same additive as that incorporated in the outer layer,preferably in a prophrtion n o t greater than that in the outer layer.

An electrode in accordance with the invention may be manufactured bypreparing a powder consisting mainly oe' ungstes and containing,dispersed throughout the powder, a siiitable'proportion of electronemissive material df low work function, placing a mass of this powder incontact with, so as to be substantially surrounded by, a second mass ofpowder consisting either of pure tungsten or of tungsten containing oneor more additives as aforesaid, pressing the assembly of powders to forma composite compact, heating the compact in a reducing atmosphere toexpelvolatile additive material, if present, and to sinte'r the compact,and subjecting the sintered composite body to the required workingoperations for forming an electrode of the desired shape and size.Usually the worked body will be in the form of a rod of considerablelength, and may be cut to suitable lengths for forming a number ofelectrodes. In some cases it may be desirable, in forming the compositebody, to surround the core completely with the outerlayer material, forease of working; then a portion of the outer layer may be subsequentlyremoved by grinding if requir'ed, either to expose a part of the core orto reduce the thickness of the outer layer. 7

The two powder masses may conveniently be arranged incontact with oneanother in a mould of suitable shape, divided into compartments by meansof one or more suitable "spacers, for the introduction of the powdersinto the mould.

The powdered materials to be used for forming the core and the outerlayer respectively may each be prepared in known manner, the electronemissive material, and other additives if used, being introduced intothe respective masses of tungsten powder by known methods,

for example by treating tungsten or tungsten oxide powder's withsolutions of suitable salts and heating the mixtu're's under reducingconditions to bring about decomposition of the salts to the requiredadditives and reduc tion of thetungsten oxide, if present, to tungsten.The methods employed for preparing the two powders are preferably suchthat the powders obtained are of the same orderof particle size and havea similar degree of porosity after being subjected to the samecompression load: these precautions will ensure that the respectiveparts of the composite body formed from the powders will 'sinter atsubstantially the same rate and shrink to substantially the same extentduring sintering, so that cracking of one part or the other duringsintering is avoided.

Usually tungsten oxide is used as the starting material in theproduction of both masses of powder, and since the particle size of theproduct is strongly influenced by the conditions under which thereduction of the tungsten oxide to tungsten is carried out, theseconditions should be so arranged, for the respective reductionprocesses, that the two powders finally obtained are as closely matchedas possible in respect of particle size and porosity. For example, it iswell known that when thoria is dispersed in tungsten a very finetungsten powder usually results so that if thoria is employed as theelectron emissive material, the reduction step in the preparation of thecore material is preferably carried out under conditions which tend toproduce larger particles, that is to say in a moist atmosphere, atrelatively high temperatures, and with a low flow rate of the reducinggas. On the other hand, since the types of tungsten powder to be usedfor the outer layer in accordance with the invention tend to be formedin larger particles, the reduction step in the preparation of thispowder is preferably carried out under conditions tending to producesmall particles, that is to say at relatively low temperatures, in anatmosphere as dry as possible, and with a high flow rate of the reducinggas.

In some cases it may be desirable, for the production of the corematerial, first to prepare a powdered master mixture of tungsten and theelectron emissive material containing a relatively high proportion, forexample 15% by weight, of the electron emissive material, and then tomix this powder with some of the powder to be em ployed for the outerlayer, to give a final powder containing the desired proportion, forexample 1 to 5% by weight, of the electron emissive material. Thismethod of preparing the core powder is advantageous in giving a moreuniform distribution of the electron emissive material throughout thecore.

One specific method of manufacturing an electrode in accordance with theinvention will now be described by way of example.

In the method of the example, a batch of the material for the core ofthe composite body from which the electrode is to be formed is preparedby thoroughly mixing 5 kilograms of tungstic oxide (W0 powder with 525mls. of an aqueous solution of thorium nitrate containing gms. of Th(NOper litre of solution. The resulting slurry is heated gently to driveoff most of the water, and the mixture is crushed and then heated inhydrogen to reduce the tungstic oxide to tungsten and to decompose thethorium nitrate to thoria: for the reduction process, the powder iscontained in boats, in batchesof 100 gms. in each boat, and the boatsare passed through a long furnace tube comprising four zonessuccessively held at temperatures of approximately 550 C., 750 C., 850C., and 900 C., the boats being introduced into the furnace tube at12-minute intervals, and the time taken for each boat to pass throughthe four temperature zones being three hours. The atmosphere in thefurnace tube is kept moist by the expulsion of residual water from thepowder, and hydrogen is arranged to flow through the tube at the rate of25 to 30 cubic feet per hour. The product of the reduction consists oftungsten powder containing a substantially uniform, finely divideddispersion of thoria (ThO in the proportion of 0.7% by weight of theweight of tungsten.

For the outer layer of the composite body, in accordance with theexample, silicated tungsten powder is employed. This powder is preparedby mixing powdered tungsten oxide with aqueous solutions of potassiumsilicate and sodium chloride, in the proportions of 34 mls. of a 5%solution of sodium chloride and 50 mls. of a 5% solution of potassiumsilicate to each kilogram of the oxide. The slurry is heated to removeas much water as possible, and the dried material is crushed andsubjected to a reduction process. In this case the reduction is carriedout in two stages: for the first stage,

the powder, which is yellow in colour, is passed through fourtemperature zones in a furnace tube, in 100-gram batches contained inboats introduced into the furnace at 12-minute intervals, while a flowof dry hydrogen is maintained through the tube at the rate of 60 cubicfeet per hour, the four successive zones being maintained attemperatures of approximately 550 C., 640 C., 660 C. and 740C. At theend of the first stage the material is only partially reduced to a brownoxide but is thoroughly dry; for the second stage of the reduction theproduct of the first stage is mixed with an equal amount of the initialyellow oxide powder, and this mixture is again passed through thefurnace tube at the same rate and with the same ratev of hydrogen flowas in the firststage, the four successive temperature zones beingmaintained at approximately 550 C., 750 C., 800 C., and 850 'C. v Theproduct of the second stage consists substantial-lyof tungsten powdercontaining a uniform dispersion of'silica, with small amounts ofpotassium oxideand sodium oxide, the total proportion of additivematerial being approximately 1% of the weight of tungsten.

"The particle sizes and porosities of both of the powders prepared asdescribed above have been determined by means of the apparatus known asthe Fisher Sub Sieve Size'r and described in United States patentspecification No. 2,261,802. When subjected to a compression load of 4091138481]. in this apparatus, the thoriated tungsten powderfwas found tohave a particle size range of up to S microns, with an averageparticlesize of 1.45 microns and the silicated tungsten powder was found to havea particle size range up to 5 microns, with an average particle size of3.39 microns, and the porosity figures obtained were 0.732 for thethoriated tungsten and 0.734 for the silicated tungsten.

The mould employed for forming the composite mass of powders is showndiagrammatically, in sectional ele v ation; in'FIGURE 1 of theaccompanying drawings. The mould comprises a steel container 1 having acavity of length corresponding to the desired length of the pressedcomposite bar, of width corresponding to the de sired width of thepressed bar and of depth about four times the width. ,A trough-shapeddouble filling funnel is'fitted into the open top of thernould cavity,the outer funnel 3 being made of brass and having a short stem, fittingcloselywi-thin the wall of the mould cavity, and the inner funnel 2which is made of thin tin plate, having a long stem arranged coaxiallywith that of the outer funnel, the width of which stern can be variedaccording to the desired width or the core of the'composite bar.

Suitable dimensions of'a mould for the manufacture of a plurality ofelectrodes in accordance with the example are as follows; the mouldcavity is 60 cm. long, 1.4 cm. wide and 5.6 cm. deep, the stem of theinner funnel is adjusted to an internal width of 7 mm., and the innerfunnel is arranged so that its's'te'm extends downwards into the mouldcavity for 4.2 cm. To form the composite bar, using a mould of thesedimensions, 380 grams of silicated tungsten powder, prepared asdescribed above, are first introduced into the mould cavity, filling itup to the level at indicated in the drawing. The double funnel is thenplaced in the position shown, and 140 grams of the thoria-ted tungstenpowder, prepared as described above, are introduced into the centralcompartment 4 of the mould cavity through the inner funnel 2, fillingthis compartment up to the level b. The outer compartments 5, 5 of themould cavity are then filled up to slightly above the level b byintroducing 250 grams of silicated tungsten powder through each sidechannel formed between the inner and outer funnels. The double funnel isthen carefully removed, avoiding disturbance of the powder, and 280grams of silicated tungsten powder are added, to fill the remainingspace in the mould cavity above the level b: the object of slightlyoverfilling the outer compartments 5, 5 is to ensure that none of thecore material can flow outwards on removal of the inner funnel.

The composite mass of powders so formed is subjected to a pressure ofapproximately 15 tons/sq. in.; if desired, to facilitate pressing, 0.5%by weight of camphor maybe added, in ether solution, to each of thetungsten powders before they are introduced into the mould.

The pressed compact is removed and heated slowly in a hydrogenatmosphere to a temperature above 1000 C. to remove the camphor, ifpresent, and to strengthen the compact for facilitating subsequenthandling. The compact is then raised rapidly, in approximately 4 to 6min ut-es, to the sintering temperature by the passage of an electriccurrent through the bar, the current being increased in that time to avalue corresponding to -93% of the current required to melt thetungsten; the current is held at this final value for 12 minutes, thetemperature being approximately 2950 C. The sintering is carried out inan atmosphere of hydrogen.

The sintered composite bar thus produced is worked down to a rod of thedesired dimensions, by hot-rolling and/ or swaging in known manner, andis then cut into the desired shorter lengths to form electrodes for arcwelding or spark-cutting apparatus, or to form cathodes for electricdischarge lamps. Each rod produced in the manner described above willhave a core of diameter approximately half the total diameter of the roditself. If desired, the total diameter, and thickness of the outerlayer, may be further reduced by grinding; also if de- 30 duced by themethod of the example is shown diagrammatically, in section, and greatlyenlarged in diameter, in FIGURE 2 of the accompanying drawings. Thiselectrode is suitable for use in inert gas shielded tungsten arc weldingapparatus operated on direct current, and in form consists of a straightrod 6 of diameter, for example, 1 to 3 mm., tape'red at the end 7 atwhich the arc is to be produced. The rod is composed of a core 8 oftungsten containing 0.7% by weight of thoria, and an outer layer 9 ofsilicated tungsten: the diameter of the core is approximately half thetotal diameter of the main body, that is to say the non-tapered part, ofthe rod; the end is tapered by grinding, so that the outer layer isreduced in thickness at this end, while the diameter of the core isconstant throughout the length of the rod.

One particular form of inert gas shielded tungsten arc welding apparatusin which an electrode of the above-described form can be employed isshown diagrammatically in FIGURE 3 of the accompanying drawings. Theapparatus comprises a welding torch 10 of known general type, supportedvertically above a metal work tablell. by means of an arm 12, which isarranged to be movable (by means not shown in the drawing) to cause thetorch to traverse the work table in a horizontal direction. Theelectrode 13 is held in a vertical position at the lower end of thetorch, and a nozzle 14 of refractory ceramic material is screwed on tothe main body of the torch so as to substantially enclose the electrode,leaving only the tip of the electrode exposed: the nozzle is shownpartly broken away to expose part of the electrode to view.

The apparatus is shown, by way of example, set up for a seam weldingoperation, the work pieces to be welded together being two metal strips15, 16 which are held in position on the work table by clamping means 118. A means, shown diagrammatically as an adjusting screw 19, isprovided for adjusting the distance between the tip of the electrode andthe junction 20 between the work pieces where the weld is to be formed.

Cables 21, 22 for connection to an electrical supply are connectedrespectively to the electrode, passing through the wall of the torchbody, and to the work table. An

inlet pipe 23 is provided for admitting a flow of inert gas into thetorch, so that in operation the gas will fill the space around theelectrode within the torch body and will flow out through the nozzle 14so as to surround the tip of the electrode and form an inert atmospherein the region of the arc formed and the metal surfaces to be weldedtogether.

Apparatus of the form shown in FIGURE 3, including an electrode of theform described above with reference to FIGURE 2, is suitable foroperation from an open circuit direct current electrical supply of 2 to600 amperes and 45 to 85 volts, giving an arc voltage of 9 to 14 volts.The inert gas employed may be, for example, argon, helium, or a mixtureof argon and helium.

We claim:

1. An electron emissive electrode for use in an electric dischargedevice, wherein the electron emissive part is formed of a one-piece,composite sintered-poWder-mass and worked body composed mainly oftungsten and consisting of a core of tungsten containing a dispersion ofelectron emissive material and, integral with the core and covering atleast the whole of the major surface of the core, which surface issubjected to direct mechanical action during working, an outer layer ofhigher ductility than the core and consisting essentially'of tungstenand free from such electron emissive material, the electron emissivematerial included in the core being such that the core has anappreciably lower Work function than that of the said outer layer.

2. An electron emissive electrode according to claim 1, wherein theelectron emissive material included in the said core consists of atleast one oxide selected from the group consisting of thorium oxide,zirconium oxide, yttrium oxide, lanthanum oxide and the rare earthoxides.

3. An electron emissive electrode according to claim 1, wherein theelectron emissive material included in the said core consists of thoriumoxide, in a proportion in the range of 0.5% to 15% by weight of thetotal weight of the core.

4. An electron emissive electrode according to claim 1, wherein the saidouter layer consists of pure tungstem.

5. An electron emissive electrode according to claim 1, wherein the saidouter layer consists of tungsten containing a non-electron emissiveadditive material incorporated in the tungsten for influencing thecrystal structure thereof.

6. An electron emissive elect-rode according to claim 1, which is in theform of a rod, wherein the whole of the lateral surface of the core iscovered by the said outer layer and the end surface of the core isexposed at least at one end of the rod, and wherein the ratio of thethickness of the said outer layer to the diameter of the core is in therange of 7:1 to 1:7.

7. An electron emissive electrode according to claim 5, wherein the saidadditive material consists of alumina.

8. An electron emissive electrode according to claim 5, wherein the saidadditive material consists of potassiumsilicate.

9. An electron emissive electrode accordingto claim 5, wherein the corealso contains the same non-electron emissive additive material as thatincorporated in the outer layer, in a proportion not greater than thatin the outer layer.

10. An electron emissive electrode for use in an electric dischargedevice, wherein the electron emissive part is in the form of a one-piececomposite rod formed by working a sintere-d-powder-mass body and.consisting of a core formed from tungsten powder containing asubstantially uniform dispersion of thoria in the proportion of 0.7% byweight of the weight of tungsten and, integral with the core andcovering the whole of the lateral surface thereof, an outer layer formedfrom tungsten powder containing 1% of its weight ofcrystallization-controlling, non-electron emissive, additive materialconsisting of silica, potassium oxide and sodium oxide, the end surfaceof the core being exposed at one end of the rod and the said end of therod being tapered, and the diameter of the core being half the totaldiameter of: the non-tapered part of the rod.

11. An electron emissive electrode in the form of a onepie'ce compositerod formed by Working a sintered-powder-mass body composed mainly oftungsten and consisting of a core of tungsten containing a dispersionofelectron emissive material and, integral with the core and covering thewhole of the lateral surface thereof, an outer layer ofhigher ductilitythan the core and consisting essentially of tungsten and free from suchelectron emissive material, the electron emissivematerial included inthe core being such that the core has an appreciably lower work functionthan that of the said outer layer, the ratio of the thickness of thesaid outer layer to the diameter of the core being in the range of 7:1to 1:7, and the end surface of the core being exposed at least at oneend of the rod.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCESGibson, Thoriated-Tungsten Electrodes, The Welding Journal Feb. 1952,pp. 102408, Sept. 28, 1960.

RICHARD D. NEVIUS, Primary Examiner.

W. L. JARVIS, Assistant Examiner.

1. AN ELECTRON EMISSIVE ELECTRODE FOR USE IN AN ELECTRIC DISCHARGEDEVICE, WHEREIN THE ELCTRON EMISSIVE PART IS FORMED OF A ONE-PIECE,COMPOSITE SINTERED-POWDER-MASS AND WORKED BODY COMPOSED MAINLY OFTUNGSTEN AND CONSISTING OF A CORE OF TUNGSTEN CONTAINING A DISPERSION OFELECTRON EMISSIVE MATERIAL AND, INTEGRAL WITH THE CORE AND COVERING ATLEAST THE WHOLE OF THE MAJOR SURFACE OF THE CORE, WHICH SURFACE ISSUBJECTED TO DIRECT MECHANICAL AC-